1. Field of the Invention
The present invention relates to a fuel cell system and a draining method for the same, based on the use of a reformer for producing a reformed gas containing hydrogen gas from a liquid mixture of methanol and water to supply the reformed gas to a fuel cell comprising an anode electrode and a cathode electrode provided opposingly with an electrolyte interposed therebetween.
2. Description of the Related Art
A fuel cell stack has been developed, which comprises, for example, a plurality of stacked fuel cells interposed by separators, the fuel cell including an anode electrode and a cathode electrode provided opposingly with a solid polymer electrolyte membrane interposed therebetween. Such a fuel cell stack has been practically used for a variety of applications.
The fuel cell stack of the type as described above is constructed as follows. That is, for example, an aqueous methanol solution is reformed with steam to produce a reformed gas (fuel gas) containing hydrogen gas which is supplied to the anode electrode, while an oxygen-containing gas (air or oxygen gas) is supplied to the cathode electrode. Thus, the hydrogen gas is ionized, and it flows through the solid polymer electrolyte membrane. Accordingly, the electric energy is obtained at the outside of the fuel cell.
In the case of the vehicle-carried type fuel cell stack, it is difficult to replenish the aqueous methanol solution, for example, especially for passenger cars which are not aimed to run on a regular route. For this reason, it is practical to use a system in which only methanol is replenished, and water produced by the reaction in the fuel cell stack is recovered and utilized as necessary water. However, it is feared that the water necessary for the reforming may be frozen when the fuel cell stack is used in the cold climates, because of the provision of a water tank having a relatively large capacity. A problem arises in that it is difficult to start up the fuel cell stack smoothly.
An apparatus for supplying the raw material liquid is known, for example, as disclosed in Japanese Laid-Open Patent Publication No. 8-91804. The apparatus comprises a methanol storage tank for storing methanol; a storage tank for storing reforming raw material liquid arranged under the methanol storage tank and connected with a supply passage for water discharged from a fuel cell, for storing a liquid mixture of methanol and water; and an inflow amount-adjusting means arranged between the methanol storage tank and the storage tank for storing reforming raw material liquid, for adjusting the inflow amount of methanol fed from the methanol storage tank to the storage tank for storing reforming raw material liquid to obtain a predetermined value of the mixing ratio of methanol and water in the storage tank for storing reforming raw material liquid.
In the case of the conventional technique described above, it is possible to avoid the freezing because no water tank is used. However, the water, which remains in the water supply passage for connecting the fuel cell and the storage tank for storing reforming raw material liquid, tends to freeze. Therefore, a problem is pointed out in that the supply passage may be closed thereby.
Further, the reforming reaction for the aqueous methanol solution is represented by CH.sub.3 OH+H.sub.2 O.fwdarw.3H.sub.2 +CO.sub.2, while the reaction in the fuel cell is represented by 3H.sub.2 +CO.sub.2 +3/2O.sub.2.fwdarw.3H.sub.2 O+CO.sub.2. In view of this fact, the water necessary to change 1 mol of methanol to hydrogen is theoretically 1 mol, while the produced water discharged from the fuel cell is 3 mols. Therefore, an excessive amount of produced water is introduced into the reforming raw material storage tank. A problem arises concerning the handling of water which is excessive with respect to the mixing ratio of methanol and water.